It is well known that ethylenically unsaturated monomers like vinyl aromatic compounds, such as styrene, .alpha.-methylstyrene, vinyltoluene or divinylbenzene or acrylic monomers, such as acrylic acid, methacrylic acid and their esters and amides, or unsaturated esters such as vinyl acetate or unsaturated polyesters have a strong tendency to polymerize when subjected to elevated temperatures. Manufacturing processes for such monomers typically include distillations or handling at elevated temperatures.
To prevent the premature polymerization of vinyl aromatic monomers during the distillation purification process, various compounds have been disclosed as polymerization inhibitors. These include elemental sulfur and many classes of organic chemicals having varying degrees of success in industrial use. These compounds include among others nitrated phenol derivatives, C- and N-nitroso compounds, nitroxyl derivatives, diphenylamines, hydroxylamines, quinones, quinone oximes and quinone alkide derivatives.
Known inhibitors of acrylic monomer polymerization include phenothiazine, hydroquinone monomethyl ether, and methylene blue. Phenothiazine, while unable to totally inhibit polymerization of acrylic monomers, is a commonly used co-additive. Recent patents claim phenylenediamines with soluble transition metal salts(U.S. Pat. No. 5,221,764) and aryl N-nitroso compounds(EP 0 522 709 A2) are active in acrylic monomer stabilization. However, them still remains a need for a compound to improve the stability of acrylic monomers during their distillation. The need exists for a stable polymerization inhibitor system which will effectively and safely prevent the premature polymerization of unsaturated monomers during distillation and purification processes, particularly if air is absent.
U.S. Pat. Nos. 4,003,800 and 4,040,911 disclose the use of quinone alkides in a styrene purification process. U.S. Pat. No. 4,032,547 describes the preparation of quinone methides from phenols by a persulfate oxidation process mediated by ferricyanide. ##STR1##
In the generic structure depicted above, groups R.sub.4 and R.sub.5 include phenyl and substituted phenyl, but such structures are not exemplified in U.S. Pat. No. 4,003,800 nor is any 7-aryl quinone methide derivative included among the 17 individually named compounds in U.S. Pat. No. 4,003,800. All 17 compounds have either no substituents or an alkyl substituent in the 7-position. The individually named compounds in U.S. Pat. No. 4,003,800 include six with unsubstituted 7-methylene groups, which are clearly too thermally unstable for practical use as industrial polymerization inhibitors in unsaturated monomers.
There is much convincing experimental evidence proving that quinone methides unsubstituted in the 7-position, i.e. compounds with an unsubstituted exomethylene group, are in fact too unstable to be even isolated at room temperature. These methylene derivatives can be prepared only as a very dilute 10-3 to 10-5 molar solutions which are stable only few days in the absence of light (See, e.g.,: P. Gruenanger in Houben-Weyl, Methoden der Organischen Chemie, Vol. 7/3B, p. 420).
Quinone methides with 7-alkyl groups also lack thermal stability to be used efficiently in the present application.
Surprisingly, quinone methides with 7-aryl substituents have now been found to be much more thermally stable than 7-alkyl derivatives. For example, the 2,6-di-tert-butyl-4-isobutylidene quinone methide B decomposed into material inactive as an inhibitor of styrene polymerization on prolonged storage at room temperature whereas 7-phenyl quinone methide C is completely stable. Another convincing piece of evidence for the inherent instability of 7-alkyl quinone methides is obtained by Differential Scanning Calorimetry (DSC), see Table 1 below. The 7-alkyl quinone methides A and B exhibited strong exothermy in J/g associated with their decomposition when heated under nitrogen, whereas the 7-phenyl quinone methide C showed no exothermy at all.
TABLE 1 ______________________________________ ##STR2## ##STR3## ##STR4## ##STR5## ______________________________________ Exo- 130 J/g 175 J/g O thermy in DSC under nitro- gen ______________________________________